1. Field of the Invention
The present invention relates to an image forming apparatus in which a photoconductor is exposed by a laser beam.
2. Description of the Related Art
There has been known, as an electrophotographic image forming apparatus, an apparatus in which a surface of a photoconductor is scanned and exposed by a laser beam. A general mechanism for scanning the laser beam includes a rotating reflection member (polygon mirror), wherein a laser beam emitted from a laser source, which is located at a fixed position, is reflected on the rotating reflection member so as to be deflected in a predetermined direction according to a change in a reflection angle caused by a rotation of the reflection member, whereby the photoconductor is scanned.
If a side face (reflection surface) of the polygon mirror is dirty due to a deposition of dusts, a reflectivity of the laser beam is reduced, which affects an image. In view of this, there has been proposed a laser exposure apparatus that detects an intensity of the laser beam, which has passed through an optical component, by means of a BD sensor (beam detecting sensor) in order to correct a laser output (see, for example, Japanese Unexamined Patent Publication No. 2002-248806).
However, it has been found that dirt on the reflection surface caused with its use does not uniformly advance all over the reflection surface. According to an experience of inventors, a leading end in a rotating direction becomes dirtier than other portions. This entails a problem that the BD sensor, which detects the laser beam reflected by the leading end of the reflection surface, cannot detect the laser beam.
FIGS. 14A to 14C are explanatory views illustrating a state of dirt on a reflection surface in a conventional image forming apparatus. FIG. 14A illustrates an initial state, in which the side face (reflection surface) of the polygon mirror 211R is not at all dirty. When the image forming apparatus is used from the state illustrated in FIG. 14A, the reflection surface is brought into a state shown in FIG. 14B, and then, into a state shown in FIG. 14C. As a period when the apparatus is used is increased (when a cumulative time of rotation of the polygon mirror 211 increases), the dirt 273 is deposited onto the reflection surface of the polygon mirror 211. As apparent from FIGS. 14B and 14C, the dirt near the leading end 212 in the rotating direction is heavier than the other portions of the reflection surface.
FIGS. 15A to 15C are an explanatory view illustrating a state in which a laser beam is deflected with a rotation of one of the reflection surfaces of the polygon mirror 211 to become a scanning beam in the conventional image forming apparatus. As illustrated in FIGS. 15A to 15C, the reflectivity of the laser beam that scans a scanning start end reduces when dirt is deposited onto a leading end 212 of the reflection surface.
The reason why the dirt 273 is selectively deposited onto the leading end 212 of the reflection surface 211R as shown in FIGS. 14A to 14C is assumed as described below. An airflow produced by the rotation of the polygon mirror causes the dirt deposited onto the reflection surface of the polygon mirror to fall. However, an excessive airflow A (see the figure below) is generated at the leading end of the reflection surface, so that the airflow becomes dead. Therefore, the dirt deposited at the leading end of the reflection surface is difficult to remove compared to the other portions.
Even if the laser output is corrected as described in Japanese Unexamined Patent Publication No. 2002-248806, the output is soon beyond a correctable range as the dirt becomes heavy. If so, the laser beam cannot be detected unless the reflection surface of the polygon mirror is cleaned.